1. Field of the Invention
The present invention belongs to the field of processes for the reutilization of thermoplastic resin scrap materials, and more particularly, to such processes employing unsorted quantities of thermoplastic resin scrap materials, which may contain two or more mutually incompatible thermoplastic resins and possibly up to 25% by weight of foreign materials.
2. Description of the Prior Art
The reutilization of individual thermoplastic resins is well known and provides finished and semi-finished articles exhibiting acceptable physical properties. French Pat. No. 1,246,108 discloses a process for converting individual thermoplastic resin scraps and mixtures of compatible thermoplastic resin scraps into placques and films. According to this disclosure, the resin is first subjected to a cleaning operation in which extraneous materials such as caps, labels, etc., are removed therefrom, and thereafter a filter and possibly a reinforcing material such as a metal web or a fibrous material is incorporated into the ground resin under the influence of heat and pressure.
It is at present known, however, that the reutilization of most mixtures of thermoplastic scrap is subject to numerous difficulties when the individual thermoplastic resins contained in the mixtures markedly differ in chemical composition as a result of different values of the cohesive energy density of the various polymers (cf., for example, D. R. Paul, C. E. Vinson and C. E. Locke, "Polym. Eng. Sci.", 12(1972),157). Such resins are said herein to be mutually incompatible (i.e. each resin is insoluble into the other, thus giving rise to a heterogeneous mixture in which each type of resin forms a dispersed phase into the other), the incompatibility manifesting itself in finished and semi-finished articles prepared from mixtures of the resins as mechanical properties (e.g. tensile, flexural and impact strength) become more and more inferior to those demonstrated by articles prepared from the poorest thermoplastic resin in the mixture when cohesive energy densities and more and more different. Incompatibility may occur not only in mixtures of chemically different thermoplastic resins such as polyvinylchloride (PVC) and polystyrene, but may also exist with resins of the same polymeric type, differing however significantly in the nature and quantity of other materials contained in the individual resin formulation. By way of example of this latter type of incompatibility, it has been found that mixtures of different PVCs used in the fabrication of mineral water bottles exhibit physical properties which are significantly inferior to those of the poorest performing PVC in the mixture. The only known example of mixtures of compatible thermoplastic resins wherein the resins contained in the mixture are chemically dissimilar are those made up of polypropylenes, polyethylenes or other polyolefins. However, such mixtures are of limited practical interest owing to their low qualities. They usually do not meet with must products specifications.
Furthermore, the market based on the reutilization of scrap resins for the manufacture of less refined articles, with or without additives, consumes only a small portion of available scrap material (cf., for example, C. W. Marynowski, "Disposal of Polymer Solid Wastes by Primary Polymer Producers and Plastics Fabricators", Report No. PV 213 444 of the Stanford Research Institute, EPA,1972). The practical possibilities of completely sorting mixtures of incompatible resins into groups of like resins is very limited from the technical or economic view. On the contrary, a separate collection of mixtures of thermoplastic resins scraps, which are likely to be incompatible, is of interest under certain conditions as, for example, for large scale manufacturing concerns. It is with such mixtures of incompatible thermoplastic resins that the interest of the process of the present invention arises.
The processes which are at present known for rendering compatible otherwise mutually incompatible thermoplastic resins employ one of the following methods: 1-incorporating so-called "compatibilizing agents" such as rubbers based on copolymers of ethylene and propylene (EPR rubbers) in the case of polyethylene-polypropylene mixtures (cf., R. Longworth and D. L. Funck, J. Appl. Polym. Sci., 10 (1966), 1612; S. Onogi, T. Asada and A. Tenaka, J. Polym, Sci., A-27 (1969), 171; R. E. Robertson and D. R. Paul, J. Appl. Poly. Sci., 17 (1973), 2579), or chlorinated polyethylenes in other cases (French Patent No. 2,111,307 and U.S. Pat. No. 3,687,873; cf., also, D. R. Paul, C. R. Locke and C. E. Vinson, Polymer Eng. Sci., 13 (1973), 202). 2-incorporating into resin mixtures, substantial quantities of more or less inert fillers such as crushed glass, sawdust, paper, etc., with or without the further addition of fibrous materials (cf., for example, Mod. Plast. Intern. June. 1972, p. 10 or Plastiques Mod. Elastom, Nov. 1972, p. 159; German Pat. No. 2,357,886,).
The resin articles obtained following each of the afore mentioned processes are mostly of very mediocre quality except where more or less suitable scraps but of a single type such as polyethylene are used (cf., for example, the so-called Kabor process in which polyethylene scraps, following a manual sorting, are filled with paper to provide materials having relatively satisfactory properties; Mechanical Handling; March 1972; Europlastics Monthly, May 1972). If mixed scrap is used as the starting material, known processes (Regal Convertor of the Plastic Recycling Company Ltd or Reverzer of the Mitsubishi Petrochemical Company) provide materials exhibiting weak mechanical properties at best comparable to those of wood and returning a relatively low price per kilogram. If better properties are sought using known processes, for example, the process referred to above employing EPR rubbers as compatibilizing agents, the cost of the articles becomes prohibitive. On the contrary, it has been observed with well-defined polymers that satisfactory conversion methods are available (cf., for example, Swiss Pat. No. 496,537; Chem. Eng. 78 (1971), No. 13 p. 56; Water and Waste Treatment, Aug. 1973, P. 9; Mod. Plast. Intern., Aug. 1973 p. 54, and Oct. 1973, p. 10), if necessary, by the addition of fresh resin (cf. for example, French Pat. Nos. 2,162,762 and 2,162,763).
Heretofore, no method has been provided for the reutilization of mixtures of mutually incompatible thermoplastic resin scraps which at once results in inexpensive finished or semi-finished articles exhibiting good physical properties.